Some fixes for Czech translation

I've noticed that Screen Options tab on Orders screen doesn't work when using Czech translation of WooCommerce. Turned out there was a string in which the `<a>` tag wasn't properly closed: `... <a href=\"%s\">zapojit na GitHub</ a>.` Upon closer investigation I've found some more strings with the same issue.

When saving the file in the latest version of Poedit, I've got an error about an invalid plural forms expression. I've replaced it with the one from the [Czech WordPress package](http://cs.wordpress.org). I also got a warning about an excess placeholder in one string and fixed that too.

SergeyBiryukov/woocommerce@8224064 addresses all of these issues. GitHub says that there are 2405 additions and 2315 deletions in the .po file, but that's just a result of re-saving it in the latest version of Poedit.